1. Field of the Invention
The present invention relates to a switch circuit to be provided to a terminal to which a positive or negative voltage is input.
2. Description of the Related Art
FIG. 4 illustrates a conventional switch circuit. The switch circuit controls whether a positive or negative voltage input to an input terminal IN is transmitted or interrupted to an internal circuit 15 in response to a signal of a switch control terminal EN.
First consider the case where a positive voltage VIN+ input from the input terminal IN is transmitted to a node B serving as an input terminal of the internal circuit 15. The signal of the switch control terminal EN is set to a VDD voltage in an active state, to thereby turn ON NMOS transistors 11 and 12. Therefore, the positive voltage VIN+ input from the input terminal IN is transmitted to the node B serving as the input of the internal circuit 15. In this case, an NMOS transistor 13 is turned OFF, and hence the NMOS transistor 13 does not affect the voltage of the node B.
Next consider the case where the positive voltage VIN+ input from the input terminal IN is not transmitted to the node B serving as the input of the internal circuit 15. The signal of the switch control terminal EN is set to a GND voltage in an inactive state. A drain of the NMOS transistor 11 has the voltage VIN+ and a gate thereof has the GND voltage, and hence the NMOS transistor 11 is turned OFF. The NMOS transistor 13 is turned ON so that a node A has the GND voltage. A drain and a gate of the NMOS transistor 12 have the GND voltage, and hence the NMOS transistor 12 is turned OFF. Therefore, the positive voltage VIN+ input from the input terminal IN is not transmitted to the node B serving as the input of the internal circuit 15.
Next consider the case where a negative voltage VIN− input from the input terminal IN is not transmitted to the node B serving as the input of the internal circuit 15. The signal of the switch control terminal EN is set to the GND voltage in the inactive state. However, the drain of the NMOS transistor 11 is applied with the negative voltage VIN− lower than the GND voltage input from the input terminal IN, and hence the NMOS transistor 11 becomes an ON state in the weak inversion region. In this case, the NMOS transistor 13 is turned ON, and hence the node A has the GND voltage rather than the input negative voltage VIN−. The drain and the gate of the NMOS transistor 12 have the GND voltage, and hence the NMOS transistor 12 is turned OFF. Therefore, the negative voltage VIN− input from the input terminal IN is not transmitted to the node B serving as the input of the internal circuit 15.
In this way, even when a negative voltage is input from the input terminal IN, the conventional switch circuit can prevent the negative voltage from being transmitted to the input of the internal circuit 15, thus preventing an erroneous operation of the internal circuit.
In the conventional switch circuit, however, when the active state of the signal of the switch control terminal EN is set to the VDD voltage, the positive voltage VIN+ input from the input terminal IN equal to or higher than a voltage (VDD−VGS−VOV) cannot be transmitted to the node B, where VDD represents a power supply voltage, VGS represents a threshold voltage (VGS>0 V) of the NMOS transistors 11 and 12, and VOV represents an overdrive voltage (VOV>0 V) necessary for reliably turning ON the NMOS transistors 11 and 12.
Further, in order to transmit a voltage equal to or higher than the voltage (VDD−VGS−VOV), for example, the power supply voltage VDD, to the node B, the active state signal of the switch control terminal EN needs to be a voltage equal to or higher than a voltage (VDD+VGS+VOV). Therefore, a booster circuit or a level shift circuit is necessary, which increases the circuit scale and the cost of products.